In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
Distributed Raman amplification is used in optical networks for the amplification of optical signals. A Raman pump laser (source) provides a high power pump beam, which is injected into an optical fibre carrying the optical signal to be amplified. This generates stimulated Raman scattering, thereby amplifying the signal.
Typically, at least two Raman pump sources are used and the light from these must be combined. Further, the light from the Raman pump sources is depolarized before injection into the optical fibre to reduce any polarization dependent gain.
Current Raman pump units therefore comprise a combiner, to combine Raman pump beams, and a depolarizer, to depolarize the light in the pump beams. Various configurations are known.